The present invention relates generally to optical amplifiers used in fiber-optics for telecommunications, cable television and other fiber-optics applications. More particularly, the invention relates to an optical fiber amplifier and method for producing an amplified broadband output from an optical signal having a wavelength in the range of 1430-1530 nm.
Because of the increase in data intensive applications, the demand for bandwidth in communications has been growing tremendously. In response, the installed capacity of telecommunication systems has been increasing by an order of magnitude every three to four years since the mid 1970s. Much of this capacity increase has been supplied by optical fibers that provide a four-order-of-magnitude bandwidth enhancement over twisted-pair copper wires.
To exploit the bandwidth of optical fibers, two key technologies have been developed and used in the telecommunication industry: optical amplifiers and wavelength-division multiplexing (WDM). Optical amplifiers boost the signal strength and compensate for inherent fiber loss and other splitting and insertion losses. WDM enables different wavelengths of light to carry different signals parallel over the same optical fiber. Although WDM is critical in that it allows utilization of a major fraction of the fiber bandwidth, it would not be cost-effective without optical amplifiers. In particular, a broadband optical amplifier that permits simultaneous amplification of many WDM channels is a key enabler for utilizing the full fiber bandwidth.
Silica-based optical fiber has its lowest loss window around 1550 nm with approximately 25 THz of bandwidth between 1430 and 1620 nm. For example, FIG. 1 illustrates the loss profile of a 50 km optical fiber. In this wavelength region, erbium-doped fiber amplifiers (EDFAs) are widely used. However, as indicated in FIG. 2, the absorption band of a EDFA nearly overlaps its the emission band. For wavelengths shorter than about 1525 nm, erbium-atoms in typical glasses will absorb more than amplify. To broaden the gain spectra of EDFAs, various dopings have been added. For example, as shown in FIG. 3a, codoping of the silica core with aluminum or phosphorus broadens the emission spectrum considerably. Nevertheless, as depicted in FIG. 3b, the absorption peak for the various glasses is still around 1530 nm.
Hence, broadening the bandwidth of EDFAs to accommodate a larger number of WDM channels has become a subject of intense research. As an example of the state-of-the-art, Y. Sun et al. demonstrated in Electronics Letters, Vol. 33, No. 23, pp. 1965-67 (1997), a two-band architecture for an ultra-wideband EDFA with a record optical bandwidth of 80 nm. To obtain a low noise figure and high output power, the two bands share a common first gain section and have distinct second gain sections. The 80 nm bandwidth comes from one amplifier (so-called conventional band or C-band) from 1525.6 to 1562.5 nm and another amplifier (so-called long band or L-band) from 1569.4 to 1612.8 nm. As another example, M. Yamada et al. reported in Electronics Letters, Vol. 33, No. 8, pp. 710-711 (1997), a 54 nm gain bandwidth achieved with two EDFAs in a parallel configuration, i.e., one optimized for 1530-1560 nm and the other optimized for 1576-1600 nm. As yet another example, H. Masuda et al. reported in Electronics Letters, Vol. 33, No. 12, pp. 1070-72 (1997), a 52 nm EDFA that used two-stage EDFAs with an intermediate equalizer.
These recent developments illustrate several points in the search for broader bandwidth amplifiers for the low-loss window in optical fibers. First, bandwidth in excess of 40-50 nm require the use of parallel combination of amplifiers even with EDFAs. Second, the 80 nm bandwidth achieved by Y. Sun et al., may be very close to the theoretical maximum. The short wavelength side at about 1525 nm is limited by the inherent absorption in erbium, and long wavelength side is limited by bend-induced losses in standard fibers at above 1620 nm. Therefore, even with these recent advances, half of the bandwidth of the low-loss window, i.e., 1430-1530 nm, remains without an optical amplifier.
It is an object of the present invention to provide an optical amplifier for a range of wavelengths between about 1430 nm and 1530 nm.
It is another object of the present invention to provide a broadband nonlinear polarization amplifier (NLPA) for generating a broadband output from an optical signal having a wavelength between about 1430 nm and 1530 nm.
It is yet another object of the present invention to provide a parallel optical amplification apparatus having a combination of the NLPA and EDFA for the almost full 25 THz bandwidth between 1430 nm and 1620 nm in the low-loss window of optical fibers.
In accordance with the invention, a broadband NLPA is implemented by using a combination of cascaded Raman amplification and either parametric amplification (PA) or four-wave mixing (4WM) in optical fibers. To achieve the broad bandwidth, one intermediate order of the Raman cascade is arranged to be at a close proximity to the zero-dispersion wavelength of an amplifying fiber. This intermediate order phase matches PA (if its wavelength is greater than the zero-dispersion wavelength) or 4WM (if its wavelength is less than the zero-dispersion wavelength). PA/4WM generates sidebands and broaden the pump band. In subsequent Raman orders, the gain bandwidth is further broadened due to the convolution of the Raman gain band with the pump band. To produce an amplified broadband signal out of the NLPA of the invention, the optical signal to be amplified must have a wavelength greater than the zero-dispersion wavelength, which in turn must be greater than the pumping wavelength from a pumping means of the NLPA.
In one embodiment, a broadband NLPA employs a 1240 nm pump and an open-loop fiber with a zero-dispersion wavelength corresponding to one of the Raman orders (e.g., either 1310 nm or 1390 nm or dispersion-flattened in between). Another embodiment uses a Sagnac Raman cavity that is pumped at either 1117 nm or 1240 nm. Feedback by the Sagnac Raman cavity reduces the required pump power, and the broadband cavity design supports much of the generated bandwidth.
The present invention also relates to a parallel optical amplification apparatus having a combination of optical amplifiers. In one embodiment, the parallel optical amplification apparatus comprises two parallel stages of NLPAs with one NLPA optimized for 1430-1480 nm and the other for 1480-1530 nm. In another embodiment, the full 25 THz of the low-loss window of approximately 1430 nm to 1620 nm in optical fibers is exploited by using a parallel combination of a NLPA of the invention and a EDFA.
NLPAs have the advantage that the gain band is set by the pumping wavelengths, and gain can be provided over virtually the entire transparency region in optical fibers (i.e., between 300 nm and 2000 nm). Moreover, because NLPAs utilize inherent properties of glass fibers, NLPAs can be used even in existing fibers by modifying the terminal ends. Hence, NLPAs are fully compatible with fiber-optic systems and can take advantage of the mature fiber-optic technologies.